That thing crawling around in my tank

Some may recall the thing that looked like some sort of bug crawling around
in my tank. It was crawling up the glass, evetually I lost track of it. From
looking at weblinks sent to me I thought it was a waterbug. This morning it
showed up again, crawling across one of my white shells. I was able to take
a real close look for a long time to try to figure out what it is and like
one poster mentioned..it appears to be a snail. But this thing is really
small. But on close inspection it appears to be a tiny snail with a fairly
large (as opposed to the snail itself) shell. Its round. The thing is, when
I first bought the tank used a few weeks ago it was dry and had been sitting
outside, the rocks that came with it in jugs were also dry as was all of the
equipment. I cleaned everything thoroughly and ran water over the rocks and
rinsed them off. So where could this snail have come from? Not from my
house, thats for sure. Mabe from the pet store in the plants I bought or is
it possible for a snail to rejuvenate something like dry brine shrimp do?